>>95924555 (OP)Because people don't know what Law means, primarily because they don't understand Chaos.
If we renamed the alignments to:
Altruistic Collectivist vs Predatory Individualist
Hierarchic Authoritarian vs Egalitarian Anarchic
Then suddenly it's a lot more clear.
A hierarchist believes that authority as a concept is valid and that certain people validly wield that authority and that valid authority should always be obeyed, it's not just a convenient social construct (neutral) or a violence-backed infringement on personal liberty disguised with a pretty self-righteous coat of paint (chaos).
So as long as Arthas's claim to authority was still valid, to Uther, he should be obeyed.
So was it valid? To a Good, for authority to be valid, it must itself be Good, i.e. it must be altruistically motivated, it can't be callously self-seeking and unsympathetic (neutral) or worse maliciously exploitative (evil).
SO. Was Arthas's order righteous? Or was it out of malice or cowardice?
Well, it was indeed an order to kill innocents. However, the context was that the entire town's grain was plague-tainted meaning that everyone within was unrescuable imminently dead or rather undead. And that it was both better for the infected to die while still human, and also that undead are an imminent threat to even more innocents.
What Arthas ordered was a mercy killing and preemptive culling of guaranteed threat to innocents, who he as a Good King is obligated to protect. LG.
It's even more cut and dry than the Gygax baby killing, because though Greyhawk Orcs are innately CE as adults, there is the SCANT possibility of reform. Maybe they find a Helm of Opposite Alignment. Maybe they find a way to sublimate those violent, rebellious predatory CE inclinations into CE productive member of society, maybe as a repossesser of loaned goods or a bounty hunter or executioner. If they can manage their temper maybe a used horse salesman.
A mindless undead is just an engine of destruction.